1. Field of the Invention
The present invention relates to a laser.
2. Related Art
There is a requirement, for example in optical telecommunications networks, for lasers having a narrow linewidth output. Such lasers can be used in, for example, wavelength division multiplex systems where closely spaced, different wavelengths are utilised to carry different communications channels, or in high bit rate systems which require narrow linewidth optical sources in order to avoid the effects of dispersion. DFB (distributed feedback) semiconductor lasers, having a diffraction grating within the semiconductor gain material, have been utilised for these applications. A disadvantage of these DFB lasers is that, due to the temperature dependence of the refractive index of the semiconductor material, their output wavelength varies as their temperature varies. This generally necessitates the use of a temperature controller, and associated electronics, to stabilise the temperature, and thus output wavelength, of DFB lasers.
This problem has been overcome, to some extent, in fibre grating lasers (FGL), in which a lensed fibre, comprising a fibre grating, is aligned with a semiconductor laser diode to form a short external cavity laser, see for example "Narrow Line Semiconductor Laser Using Fibre Grating", Bird et al, Electronics Letters, Jun. 20, 1991, Vol. 27, No. 13, pp1115-1116. In this device, the laser diode facet closest to the lensed fibre end is anti-reflection coated so that lasing occurs between the back facet of the laser diode and the grating within the fibre. Since the majority of the laser cavity, including the grating, is now within the fibre, rather than the semiconductor material, the change in output wavelength of the FGL, with temperature, is far lower than that of a DFB laser.
The present applicants have found that known FGLs are not suitable for use in practical optical telecommunication networks due lo instabilities in both their output wavelength, and their output power, as either the drive current, or the temperature, of the FGL is varied.